When AJAX is used on your website an ordinary page load trigger is not adequate to use as the page is not loading. Most of the time only a portion of the page will be updated and this trigger allow you to select the portion of the page that is updating with the on-site widget. When you have selected the element that changes on the page, the trigger will be looking for any update on this specific location and when it sense any content update the execution will take place.
Please be aware that if the content change on your page takes unusual long time the trigger will not execute (generally more than 1 second). If this is the case there are other solutions to this. Please contact Clientcare.